Tree Branches Make Very Effective Improvised Water Filters, Research Finds

A very low-tech, but highly effective, new approach to water filtration has been developed by researchers at MIT. Using nothing but a small piece of sapwood, which you can find just about anywhere, you can filter out more than 99% of the dangerous microorganisms in an available source of water.

The water filtration approach can produce up to four liters of safe drinking water a day, just with the one piece of sapwood. That’s enough to get by on, if need be,

The approach works thanks to the minute size of the pores in sapwood, which allow water through, but not much else — the xylem tissue in sapwood is responsible for transporting sap up and down a tree.

“Today’s filtration membranes have nanoscale pores that are not something you can manufacture in a garage very easily,” states researcher Rohit Karnik, an associate professor of mechanical engineering at MIT. “The idea here is that we don’t need to fabricate a membrane, because it’s easily available. You can just take a piece of wood and make a filter out of it.”

There are a number of water-purification technologies on the market today, although many come with drawbacks: Systems that rely on chlorine treatment work well at large scales, but are expensive. Boiling water to remove contaminants requires a great deal of fuel to heat the water. Membrane-based filters, while able to remove microbes, are expensive, require a pump, and can become easily clogged.

Sapwood may offer a low-cost, small-scale alternative. The wood is composed of xylem, porous tissue that conducts sap from a tree’s roots to its crown through a system of vessels and pores. Each vessel wall is pockmarked with tiny pores called pit membranes, through which sap can essentially hopscotch, flowing from one vessel to another as it feeds structures along a tree’s length. The pores also limit cavitation, a process by which air bubbles can grow and spread in xylem, eventually killing a tree. The xylem’s tiny pores can trap bubbles, preventing them from spreading in the wood.

“Plants have had to figure out how to filter out bubbles but allow easy flow of sap,” Karnik notes. “It’s the same problem with water filtration where we want to filter out microbes but maintain a high flow rate. So it’s a nice coincidence that the problems are similar.”

For the research, Karnik collected branches of white pine, and stripped them of their bark. Small sections were then cut, each around an inch long and half an inch wide. Each of these was then fixed with plastic tubing, and sealed with epoxy and clamps.

Before experimenting with contaminated water, the group used water mixed with red ink particles ranging from 70 to 500 nanometers in size. After all the liquid passed through, the researchers sliced the sapwood in half lengthwise, and observed that much of the red dye was contained within the very top layers of the wood, while the filtrate, or filtered water, was clear. This experiment showed that sapwood is naturally able to filter out particles bigger than about 70 nanometers. However, in another experiment, the team found that sapwood was unable to separate out 20-nanometer particles from water, suggesting that there is a limit to the size of particles coniferous sapwood can filter.

Finally, the team flowed inactivated, E. coli-contaminated water through the wood filter. When they examined the xylem under a fluorescent microscope, they saw that bacteria had accumulated around pit membranes in the first few millimeters of the wood. Counting the bacterial cells in the filtered water, the researchers found that the sapwood was able to filter out more than 99% of E. coli from water.

The researchers note that while the sapwood works very effectively as a filter for most types of bacteria, it will probably be largely ineffective for filtering out viruses — on account of their far smaller dimensions.

The researchers are now planning to test the sapwoods from a variety of different types of trees.

There are also a number of kinks to be worked out. For example, “designers interested in using sapwood as a filtering material will also have to find ways to keep the wood damp, or to dry it while retaining the xylem function. In other experiments with dried sapwood, the water either did not flow through well, or flowed through cracks, but did not filter out contaminants.”

“There’s huge variation between plants,” Karnik notes. “There could be much better plants out there that are suitable for this process. Ideally, a filter would be a thin slice of wood you could use for a few days, then throw it away and replace at almost no cost. It’s orders of magnitude cheaper than the high-end membranes on the market today.”

About the Author

James Ayre 's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy. You can follow his work on Google+.

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